If you use 2 rail wheelsets, and keep the insulated wheels on one side, you don't need to insulate the frame. As long as the wheels on one side are insulated from the frame, there will be no short. The frame will be "live" for whichever side you have the uninsulated wheels on but that is not a problem if you use plastic Kadee boxes for the couplers. Or just use plastic Kadee couplers.
I Tend to like the metal kadees better so I use metal couplers in plastic draft gear boxes.